1. Field of the Invention
This invention relates to image forming techniques involving image transfer using an intermediate transfer medium.
2. Description of the Related Art
Wet electrophotography using liquid toner has recently been revalued because of its advantages over dry electrophotography. The advantages include high definition comparable to that of offset printing by use of minute toner particles of submicron size, the reduction in copying cost using a small amount of liquid toner, providing sufficient image density, and energy saving through the deposition of images of toner particles onto a recording medium (recording paper) at a relatively low temperature.
A direct electric field transfer system has conventionally been the mainstream of wet electrophotography using liquid toner, in which a liquid toner image formed on a photoconductor is directly transferred to a recording medium (recording paper) by means of an electric field.
However, the conventional direct electric field transfer system has such disadvantages as being likely to cause transfer unevenness due to electric field variations according to unevenness in the surface of a recording medium (recording paper) and to cause transfer failure due to variations in electrical properties and the environment dependency of the recording medium (recording paper). Thus the conventional direct electric field transfer system significantly lowers the quality of transferred images. The conventional direct electric field transfer system requires a given amount of solvent interposed between a latent image carrier and a recording medium (recording paper) so that charged toner particles electrophoretically migrate in the solvent in a known manner to be transferred to the recording medium (recording paper). As a result, a large amount of solvent adheres to the recording medium (recording paper) after transfer. The adhering solvent partially evaporates in a heat fusing process and is discharged outside the image forming apparatus. This causes such a problem of adversely affecting the human body through inhalation of the effluvium or vapor.
In order to overcome the defects of the direct electric field transfer system, provided is an indirect transfer system using an intermediate transfer medium. In a known example of the indirect transfer system, an image of toner particles is transferred from a latent image carrier to an intermediate transfer medium under an electric field, and then is transferred to a recording medium (recording paper) by means of pressure (and heat). Another example of the indirect transfer system uses pressure (and heat) for both the transfer of an image of toner particles to an intermediate transfer medium and the transfer of the image of toner particles to a recording medium (recording paper), without using electric fields.
Either of the examples uses pressure to transfer an image of toner particles dried to a recording medium (recording paper) avoiding adhesion of solvent to the recording medium (recording paper), and thereby highly efficiently transferring the image of toner particles to the recording medium (recording paper). Those examples can adjust pressure to transfer images of toner particles to various kinds of record media (recording papers), being advantageously versatile in paper.
The conventional indirect transfer system of transferring images of toner particles to a recording medium (recording paper) by means of pressure, generally uses a rubber material having elasticity for an intermediate transfer medium. The surface of the intermediate transfer medium thus has some degree of adhesion. This can cause the recording medium (recording paper) to adhere to and entangled with the intermediate transfer medium during the transfer of images of toner particles from the intermediate transfer medium to the recording medium (recording paper) in the conventional indirect transfer system.
In such a state, even with a separating claw or the like used to detach a recording medium (recording paper) from the intermediate transfer medium, strong adhesion of a certain kind of record medium (recording paper) to the intermediate transfer medium causes difficulty in detachment, leading to breakage of the recording medium (recording paper) even detached. A recording medium (recording paper) with a highly smooth surface especially has strong adhesion to the intermediate transfer medium, resulting in difficulty in paper feeding.
To deal with this, a recording medium (recording paper) may be chucked with another mechanism in advance to prevent entanglement with the intermediate transfer medium. This, however, disadvantageously complicates the mechanism.